The invention relates to crystalline gallium nitride. In particular, the invention relates to a homoepitaxial gallium nitride based photodetector and a method of producing the same.
During the past decade there has been tremendous interest in gallium nitride (GaN) based optoelectronic devices, including, for example, light emitting diodes (LEDs) and laser diodes (LDs). Because high-quality GaN substrates have not been available, virtually all of the art has involved heteroepitaxial deposition of GaN and GaInAlN on sapphire or SiC substrates. A thin low-temperature buffer layer, typically AlN or GaN, is used in order to accommodate the lattice mismatch between GaN and the substrate and maintain an epitaxial relationship to the substrate.
Several processes are currently used to produce crystalline gallium nitride substrates. The processes include heteroepitaxial growth of gallium nitride on a substrate, such as a sapphire or silicon carbide. The heteroepitaxial growth process often results in defects including high concentrations of dislocations, vacancies, or impurities. These defects may have undesirable and detrimental effects on epitaxially grown gallium nitride, and may adversely influence operation of the resultant gallium nitride-based device. These adverse influences include compromised electronic performance and operation. Presently, heteroepitaxial gallium nitride growth processes require complex and tedious steps to reduce defect concentrations in the gallium nitride.
Known growth processes do not provide large gallium nitride crystals of high quality (i.e.; crystals having low dislocation densities); for example, gallium nitride crystals greater than about 0.8 inches (about 2 centimeters) in diameter or greater than about 0.01 inches (about 250 microns) in thickness. Further, the known methods are not known to provide for production of large gallium nitride crystals that result in single-crystal gallium nitride boules, for example gallium nitride crystals of about 1 inch in diameter and about 0.5 inches in thickness, which are suitable for forming wafers. Thus, applications for gallium nitride are limited due to size constraints.
Known methods of producing large-area GaN wafers yield wafers having rather high ( greater than 106 cmxe2x88x922) concentrations of threading dislocations. As is the case in heteroepitaxial devices, high concentrations of such defects degrade device performance.
Also, most known gallium nitride crystal production processes do not provide high-quality gallium nitride crystals with low concentrations of impurities and dislocations with adequate size and growth rates that are acceptable for device applications. Further, the known gallium nitride crystal production processes are not believed to provide an economical process having nitride growth rates that enable moderate-cost gallium nitride crystal production. Therefore, applications for gallium nitride are further limited due to quality and cost-of-production factors.
Use of gallium nitride crystal has been limited in photodetector applications because of the quality and manufacturing issues discussed above. A high-performance photodetector could be used, for example, to control the temperature in the combustor of power-generation turbines or in aircraft engines, allowing continuous, real-time optimization of combustion conditions and improved energy efficiency and reliability. Photodetectors could also be used in a wide variety of sensor applications, both civilian and military. While current buffer-layer technology allows for production of commercially viable GaN-based LEDs and LDs, the photodetectors that can be produced with current technology are marginal in performance because of very high defect levels.
Growth of homoepitaxial photodetectors on high-quality GaN substrates would offer improved sensitivity, increased efficiency, reduced leakage (dark) current, and increased breakdown field. Other potential benefits of homoepitaxial photodetectors include increased temperature of operation, better reliability, better device uniformity, improved backside contact capability, higher manufacturing yield, longer lifetime, enhanced wafer utilization, improved wavelength selectivity, and better manufacturability.
Accordingly, there is a need in the art for an improved GaN based photodetector.
The present invention meets this need and others by providing a photodetector having a gallium nitride substrate, a gallium nitride substrate for a photodetector device, and a method of producing such a photodetector.
The photodetector of the present invention includes a gallium nitride substrate, at least one active layer disposed on the substrate, and a conductive contact structure affixed to the active layer and, in some embodiments, the substrate. In one embodiment of the invention, the photodetector has a metal-semiconductor-metal (MSM) type structure, in which an insulating active layer is deposited on the gallium nitride substrate, and the conductive contact structure is a patterned array of interdigitated Schottky-type (i.e., rectifying) metallic contacts connected to the semi-insulating active layer.
Another embodiment of the invention is a photodetector having a P-i-N structure. The photodetector includes either an n-doped gallium nitride substrate or an n-doped active layer deposited on the substrate, an insulating active layer, and a p-doped active layer. In this embodiment, the conductive contact structure comprises at least one ohmic-type contact connected to the p-type active layer and an ohmic contact connected to the substrate.
The photodetector of the present invention also encompasses a third embodiment, which is a Schottky-barrier structure, in which an insulating active layer is deposited on the gallium nitride substrate, and the conductive contact structure comprises at least one Schottky-type contact connected to the insulating active layer and an ohmic contact connected to the substrate.
Accordingly, one aspect of the present invention is to provide a photodetector comprising: a gallium nitride substrate; at least one active layer disposed on the gallium nitride substrate; and at least one conductive contact structure affixed to at least one of the gallium nitride substrate and the active layer.
A second aspect of the present invention is to provide a gallium nitride substrate for a photodetector. The gallium nitride substrate comprises a single crystal gallium nitride wafer and has a dislocation density of less than about 105 cmxe2x88x922.
A third aspect of the present invention is to provide a photodetector. The photodetector comprises: a gallium nitride substrate, the gallium nitride substrate comprising a single crystal gallium nitride wafer and having a dislocation density of less than about 105 cmxe2x88x922; at least one active layer disposed on the gallium nitride substrate, the active layer comprising Ga1xe2x88x92xxe2x88x92yAlxInyN1xe2x88x92zxe2x88x92wPzAsw, wherein 0xe2x89xa6x, y, z, wxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61, and 0xe2x89xa6z+wxe2x89xa61; and at least one conductive contact structure affixed to at least one of the gallium nitride substrate and the active layer.
A fourth aspect of the invention is to provide a method of making a photodetector, the photodetector comprising a gallium nitride substrate, at least one active layer disposed on the gallium nitride substrate, and at least one conductive contact structure affixed to at least one of the gallium nitride substrate and the active layer. The method comprises the steps of: providing a gallium nitride substrate; depositing at least one active layer on the gallium nitride substrate; and affixing a conductive connecting structure to at least one of the at least one active layer and the gallium nitride substrate.
These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.